1. Field of the Invention
The present invention relates to the treatment of solid containing material derived from effluent, especially to recover useful materials therefrom.
In particular, this invention is concerned with a method for recovering useful materials from what has hitherto been regarded as the waste material produced by an industrial plant for treating paper, especially for de-inking waste paper.
Paper and paperboard products are generally made by preparing a dilute aqueous suspension containing cellulosic fibres derived from wood or from some other suitable fibrous material, and de-watering the suspension on a moving wire mesh belt to form a sheet material. The suspension may also contain a proportion of a mineral filler material which improves the brightness and opacity of the finished paper or board. Natural cellulosic fibrous materials are generally expensive, and the trees or other plants from which they are obtained must be replaced if the balance of the environment is not to be disturbed. It is therefore desirable to incorporate in the aqueous suspension from which the sheet material is made a substantial proportion of cellulosic fibrous material which has been recovered from waste paper. Most waste paper which is recycled has been printed on at least one of its surfaces, and it is necessary to separate the printing ink residue from the cellulosic fibres if the recovered material is to be suitable for use as a starting material in a paper making process. If the ink is not separated, the fibrous material will generally be unacceptably dark in colour. Generally, printing ink contains elemental carbon (carbon black) and other dark material which causes the dark coloration.
Commercially operated processes for de-inking waste paper generally comprise a pulping stage in which the waste paper is subjected to mechanical agitation in water which also contains sodium hydroxide which causes the fibres to swell, and which breaks down most ink vehicles by saponification or hydrolysis, sodium silicate which acts as a pH buffer and agglomerates detached ink particles to a convenient size, and a surfactant which wets the ink particles and helps to keep them in suspension. The suspension formed in the pulper is passed through a primary screening system in which heavy foreign bodies, such as staples, paper clips and pieces of grit are removed. The suspension passing through the screens is then fed to a treatment plant comprising one or more froth flotation cells, or one or more washing units, or a combination of washing units and flotation cells. The froth flotation cells are each provided near the bottom with a rotating impeller and means for admitting air under pressure in the form of fine bubbles in the region of the impeller. It may also be advantageous to add to the suspension, before it enters the flotation cell, a reagent known as a collector which attaches itself preferentially to the ink particles and increases their affinity with air relative to that with water. As a result, the ink particles are preferentially lifted by the air bubbles to the surface of the suspension in the flotation cell, where they are discharged in a froth product.
In the washing units the pulp is subjected to agitation in fresh or recycled water, optionally containing a dispersing agent, and the washed pulp is then drained on a screen of aperture size such as to retain the relatively long cellulosic fibres, while allowing to pass the mineral and organic particles and the fine fibre fragments.
The suspension which is discharged from the bottom of the flotation cell, and/or the suspension which passes the screens of the washing steps, is de-watered and the de-watered material, which consists of substantially de-inked cellulosic fibre material, may be subjected to further purification steps before being finally de-watered and dried for re-use in a sheet forming process.
The de-inking treatment plant reject consists of an aqueous effluent or sludge which is produced in large volumes. The reject from some plants is discarded and transferred to a waste disposal site, eg a landfill site. The reject from other plants is incinerated and this beneficially reduces the volume of the reject and generates useful heat energy from the combustion of organic components. The ash from the incineration is subsequently disposed of. However, in both cases, there is an undesirable cost and environmental impact associated with disposal. Also, potentially useful materials are wasted by the disposal process. The de-inking treatment plant, reject generally contains, in addition to the ink particles and various additives employed in paper, a substantial proportion of the inorganic filler particles which were originally present in the waste paper. These filler particles usually consist predominantly of a mixture of kaolin clay and calcium carbonate in various proportions, although other inorganic filler particles such as silicates, talc, calcium sulphate or titanium dioxide may also be present in minor proportions.
2. Related Prior Art
In the prior art, various patent specifications have been published which describe processes for incinerating waste solids from de-inking sludge. In some of these processes the ash produced following incineration is further treated to render the particulate material contained in the ash suitable for re-use in a paper making process. Examples of specifications describing such processes include WO96/32354. Where the sludge solids treated include calcium carbonate this can react with kaolin present to produce a glassy composite phase which causes the particulate product to be abrasive. As described in EP798268-A1, the incineration temperature may be controlled so as to minimise formation of the glassy phase, but this procedure can be relatively complicated and expensive to operate and may cause the resulting particulate product to have inferior optical properties, eg a reduced brightness.